Oxidation of sulfide pulping liquor to form polysulfide liquor in situ

ABSTRACT

A MIXTURE OF BLACK LIQUOR AND WHITE LIQUOR IS OXIDIZED IN THE PRESENCE OF WOOD TO BE PULPED, SO AS TO FORM A POLYSULFIDE PULPING LIQUOR WHICH ACTS IMMEDIATELY UPON THE WOOD. THE PROCESS ALLOWS INCREASED YIELDS OF PULP WITH LOWER SULFIDITY LEVELS AND, THUS, LESSENED POLLUTION.

OXIDATION OF SUL FIDE PULPING LIQUOR TO FORM POLYSULFIDE LIQUOR IN SITU Filed April 1, 1971 March 27, 1973 R G R ER 1 3,723,242

l l l I I l INVENTOR ,IP/CHAPD 6. BARKER ATTORN United States Patent 3,723,242 OXIDATION OF SULFIDE PULPING LIQUOR TO FORM POLYSULFIDE LIQUOR IN SITU Richard G. Barker, Princeton Junction, N.J., assignor to Union Camp Corporation, Wayne, NJ. Filed Apr. 1, 1971, Ser. No. 130,320 Int. Cl. D21e 3/02 US. Cl. 162-38 7 Claims ABSTRACT OF THE DISCLOSURE A mixture of black liquor and white liquor is oxidized in the presence of wood to be pulped, so as to form a polysulfide pulping liquor which acts immediately upon the wood. The process allows increased yields of pulp with lower sulfidity levels and, thus, lessened pollution.

BRIEF SUMMARY OF THE INVENTION Various processes have been developed for pulping of wood employing alkali metal polysulfide as a primary pulping agent. For example, a process for forming a polysulfide pulping liquor is described and claimed in my US. Pat. No. 3,470,061, assigned to the same assignee as the present invention. In that patent, various manganese compounds are employed for oxidation of alkali metal sulfide to alkali metal polysulfide.

A further process for forming polysulfide pulping liquors is described in US. Pat. No. 3,216,887 where the polysulfide is formed by the air oxidation of sodium sulfide in a mixture of white liquor and black liquor. Subsequent to formation of the polysulfide, the liquor contacts the wood to be pulped. Because of this method of operation, a sulfidity level of at least 40% is necessary in order to obtain any significant yield increase and, because of this, there are significant sulfur losses. The sulfur losses provide a significant level of pollution.

White liquor, recirculated from wood impregnation, is treated so as to form polysulfide according to Domtar Canadian Pat. No. 815,432. Efficiency levels operating in accordance with this patent are relatively low.

In accordance with the present invention, it has been determined that a mixture of black liquor and white liquor can be oxidized to produce alkali metal polysulfide, while the mixed liquor is in the presence of the wood to be pulped. Operating in this manner, the relatively unstable polysulfide acts upon the wood immediately after formation. Thus, not only are there improved pulping yields, but, in addition, a lower level of sulfidity can be employed. This lower sulfidity results in significantly decreased amounts of sulfur loss, which significantly reduces pollution problems.

BRIEF DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, wood chips are impregnated with a mixture of white liquor and black liquor. Air, or another oxygen bearing gas, is introduced into the liquor in the digester, during the impregnation, in order to oxidize the alkali metal sulfide present in the mixed liquors to alkali metal polysulfide. In the oxidation of alkali metal sulfide, not only is alkali metal polysulfide produced, but, additionally, sulfur, sulfites, and thiosulfates are produced. The sulfur combines with a sulfide to yield the desired polysulfide. However, because of the instability of the polysulfide, it may react with the sulfite to produce thiosulfate. As the thiosulfate is not effective in pulping, the result of this reaction is loss of 3,723,242 Patented Mar. 27, 1973 effective pulping power, so that higher sulfur levels must be maintained in order to provide a given yield level. When these higher sulfur levels are maintained, there is an increased loss of sulfur products to the atmosphere, meaning increased pollution.

According to the present process, where the mixture of black liquor and white liquor is oxidized, in the presence of wood to be pulped, the polysulfide generated is immediately available to react with the wood. For this reason, the polysulfide is effectively employed before any substantial amount reacts with the sulfite, resulting in the ineffectual thiosulfate and loss of the yield improvement provided by the polysulfide. Thus, a given yield level can be obtained with a lower total sulfur content.

The alkali metal sulfide employed is, preferably, sodium sulfide. This is the preferred material because of its significantly lower cost and easier recovery.

In accordance with the present invention, the sulfidity level of the mixed liquor need not be above 30%. The mixed liquor generally contains between 30 and 75% white liquor, with the remainder black liquor. Preferably, between and 60% white liquor is employed. The black liquor which is used is the spent liquor from the digesting step, as is normally obtained with Kraft processing.

It was previously noted that the Landmark patent did disclose an oxidation of a black liquor-white liquor mixture to form a polysulfide for pulping. However, as also indicated, the polysulfide is unstable and thus reacts with the sulfite to form thiosulfate, before the polysulfide can effectively be used. The present process avoids this problem, by carrying out the oxidation in the presence of the wood chips, thus allowing for immediate ultilization of the polysulfide and minimizing the formation of the ineffectual thiosulfate.

As also indicated, the Canadian Domtar patent teaches the oxidation of white liquor, but this oxidation is not carried out in the presence of the wood chips. The oxidation, according to Domtar, is carried out in a vessel separate from the chips, so that the immediate reaction of the polysulfide with the wood is not possible and, therefore, a higher total sulfur level is required in order to obtain a given yield level, some of the benefits of the polysulfide being lost. The amount of oxygen bearing gas which is employed, according to the process of the present invention, should be sufiicient to oxidize a significant I quantity of the sulfide present in the mixed liquor to polysulfide. Generally, suflicient polysulfide is produced to significantly improve the yields employing air in an amount of from 200 to 2,000 standard cubic feet for each ton of pulp being processed. The gas being employed in this oxidation can be any having an oxygen content of from 15 to substantially the remainder of the gas stream being made up of chemically inactive gases.

During impregnation of the wood, the temperature should be maintained below C. and, preferably, the wood chips are impregnated with the mixed liquor at a temperature of about 90 C. for about 45 minutes, While maintaining the flow of the oxygen bearing gas. The flow of oxygen bearing gas is then terminated and the temperature of the contents are raised to normal pulping temperatures of about to C. in order to complete the pulping stage.

A primary advantage of the present process is that the'formation of polysulfide, with increased yields, can be carried out employing pulping solution having sulfidity levels no greater than 30%. Thus, the white liquor employed contains between 5 and 60 grams per liter of sulfur, in all of its compound and elemental forms. Preferably, the amount of sulfur contained in the white liquor is between 10 and 30 grams per liter. The amount of alkali contained in the pulping solution is not critical,

but should range from 15 to 400 grams per liter, as Na O. More preferably, the total alkali content is from 70 to 120 grams per liter. The alkali is generally derived from the group consisting of sodium hydroxide, sodium carbonate, and sodium sulfide.

Paper produced according to the process of the present invention, compared with paper produced by the standard Kraft process at equivalent Kappa numbers, does not show any difference with regard to burst or tear strength. There is a slight increase in the luminous reflectance of these paper products, when compared with the Kraft process using a black liquor recycle.

The improvements accomplished by the process of the present invention will be described in greater detail in the following examples. These examples should be considered as illustrative only, and not as limiting in any way the full scope of the invention as covered in the appended claims.

EXAMPLE 1 Two ZO-pound charges (oven dry weight) of the same batch of slash pine were pulped according to this example. One of the charges was pulped according to a standard Kraft process, with black liquor recycle, while the other batch was pulped according to the present process where the mixed liquor contained 55% white liquor. In each case, the sodium sulfide level of the white liquor was 40 grams per liter. A quantity of cubic feet of air per hour was introduced into the wood chip-mixed pulping liquor of the charge pulped according to the present invention during the 90 minute impregnation, while maintaining the liquor at a temperature of 90 C. The digestions were otherwise carried out under the same con ditions including heating to 173 C., after impregnation, over a period of 30 minutes, and then holding the material at 173 C. for 90 minutes. The active alkali content of the White liquor employed was about 106 grams per liter, expressed at Na O. The results of pulping these two batches according to the processes indicated are shown below in Table 1:

TABLE I Screened Yield yield Kappa Type of process (percent) (percent) number Kraft process 57. 0 45. 3 83. 2 Oxidation of liquor in the presence of wood chips 58. 2 50. 2 78. 7

EXAMPLE 2 The process of the present invention was repeated, employing the same materials and conditions as noted in Example 1, but with certain process variations. In particular, the mixed liquor contains 60% white liquor and the air was introduced at a rate of 3 cubic feet per hour for a 45 minute impregnation period at 80 C. Following digestion, a yield of 57.3% and a screened yield of 51.5% were realized at a Kappa number of 72.8. Thus, the economic advantage of the process of the present invention, this advantage being obtained without an increase in pollution level, is again demonstrated.

The figure shows yield increases obtained employing the process of the present invention, when compared with standard Kraft pulping. As can be seen from the figure, at Kappa numbers of from 50 to 80, yield increases of from 1 to 3% are obtained employing the process of the present invention. These yield increases are obtained without an increase in pollution level.

While slash pine has been indicated as the wood pulped in the specific examples, various other wood species may also be used. In general, any wood pulpable according to a Kraft process can be pulped according to the present invention, with improved yields.

The various streams in the pulping process according to the present invention are recovered by the same methods normally employed in Kraft processing. The sulfide level of the black liquor leaving the digester according to the present invention is no higher than that found in the black liquor from a standard Kraft process. Thus, the present process provides a significant economic advantage over a standard Kraft process in that, while essentially the same equipment and processing are employed, increased yields are obtained.

The invention has been described in detail with particular reference made to various preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A process for the polysulfide pulping of wood comprising:

(a) impregnating the wood to be pulped with white liquor containing sulfide;

(b) adding to said white liquor-wood mixture a quantity of black liquor containing sulfide;

-(c) introducing an oxygen bearing gas to said mixture so as to oxidize the sulfide contained in the white liquor-black liquor mixture to polysulfide, while the mixture is in contact with the wood to be pulped; and

(d) digesting said wood in said oxidized white liquorblack liquor mixture.

2. The process of claim 1 wherein said oxygen bearing gas contains from 15 to 100% oxygen.

3. The process of claim 1 wherein the white liquor constitutes from 30 to of the white liquor-black liquor mixture.

4. The process of claim 1 wherein the white liquor constitutes from about 50 to 60% of the white liquor-black liquor mixture.

5. A process for pulping wood employing polysulfide as the major pulping agent comprising:

(a) impregnating the wood to be pulped with a mixture of white liquor and black liquor containing sulfide values;

(b) introducing an oxygen bearing gas into said white liquor-black liquor mixture so as to oxidize the sulfide values present in the mixture to polysulfide;

(c) digesting said wood in said oxidized white liquorblack liquor mixture.

6. The process of claim 5 wherein the white liquor constitutes from 30 to 75 of the white liquor-black liquor mixture.

7. The process of claim 5 wherein the white liquor constitutes from about 50 to 60% of the white liquor-black liquor mixture.

References Cited UNITED STATES PATENTS 3,216,887 11/1965 Landmark. 2,944,928 7/ 1960 Kibrick et a1.

S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R. 162-42, 82 

